In the early decades of the twentieth century the question of mental disabilities was widely discussed in Italy, while the first special schools for the intellectually impaired were set up. An important role was played by the Franciscan friar Agostino Gemelli (1878-1959), a physician, renowned psychologist, and founder in 1921 of the Catholic University of Milan. Gemelli promoted relevant psychological research on intellectual disabilities, based on empirical and measurable processes. He considered only scientific studies necessary to promote appropriate educational actions. Gemelli and his collaborators developed from a biological point of view the classification elaborated by the famous psychologist and psychiatrist Sante De Sanctis (1862-1935). Moreover, in 1926 Gemelli established the School for special teachers and assistants for disabled children in the Catholic University of Milan, with the aim of making the "special" teachers confident with medical, psychological and pedagogical issues. This institution did not have a "Catholic colour": indeed, Gemelli called professors of great fame to teach in his School, without concern for their ideological thoughts. All his work was characterised by the cooperation between science and religion: the experimental method did not contrast with Catholic values. This paper is based on unpublished documents from different archives.